Method of producing closed mold polyurethane foam moldings free of surface voids by flushing the mold with a vaporized halogenated hydrocarbon

ABSTRACT

Foamed polyurethane moldings free of surface voids and having a substantially uniform skin are formed in a closed mold which has been purged of air with a vaporized liquid halogenated hydrocarbon capable of condensing on the mold walls or dissolving in the molding composition at the temperature and pressure conditions used in molding.

United States Patent Harper et al. 5] May 30, 1972 [54] METHOD OFPRODUCING CLOSED [56] References Cited MOLD POLYURETHANE FOAM MOLDINGSFREE OF SURFACE vows U1 I ED STATES PATENTS BY FLUSHING THE MOLD WITH A3,178,490 4/1965 Petrino ..264/41 3,182,104 5/1965 Cwik ..264/54 xVAPORIZED HALOGENATED 3,473,951 10/1969 06116531 ..26o 25 AZ HYDROCAR UN3,476,841 11/1969 1316mm .l64/48 x Inventors: Robert C. Harper,Loveland, Ohio; Cincinnati Milling Machine Company, Cincinnati, OhioFiled: Nov. 24, 1969 Appl. No.: 879,507

U.S. Cl ..264/39, 260/25 AZ, 260/2.5 BD, 264/48, 264/51, 264/53, 264/85,264/D1G. 14

Int. Cl ..B29d 27/00 Field of Search ..260/2.5 AZ; 264/39, 51, 53,264/54, 85

Primary ExaminerJulius Frome Assistant Examiner-Paul LeipoldAtt0meyPlumley and Tyner [5 7] ABSTRACT 11 Claims, No Drawings METHOD OFPRODUCING CLOSED MOLD POLYURETHANE FOAM MOLDINGS FREE OF SURFACE VOIDSBY FLUSHING THE MOLD WITH A VAPORIZED I-IALOGENATED HYDROCARBON Thisinvention relates to the production of polyurethane foam moldings in aclosed mold. More particularly, this invention relates to the productionof foam polyurethane moldings produced in a closed mold and having asubstantially uniform skin which is void-free.

Foam plastics, especially polyurethane foams, have found increasing usein a variety of applications. For many uses, the cellular surface iseither intolerable or undesirable. Methods have been developed,therefore, to produce polyurethane foam moldings having a smooth, dense,surface skin. In one of these methods a closed mold is used. A reactivepolyurethane foam composition containing a volatile fluorocarbon blowingagent is charged to the mold, partially filling the mold to the pourline. The polyurethane mix is allowed to react exothermically, causing avolatilization of the blowing agent and thereby producing a foam whichexpands to all the previously unfilled portion of the mold above thepour line. The walls of the mold are maintained at a temperature abovethat of the original mix but below that of the exotherming polyurethanereaction mix. It is theorized that the lower temperatures at the surfaceof the mold eliminate or substantially diminish the tendency of theblowing agent to volatilize, therefore causing at the mold-moldinginterface a thin layer of polyurethane which is not blown and,consequently, resembles a dense layer or skin. Polyurethane moldings areknown to be good heat insulators and therefore this cooling andconsequent skinning effect occurs primarily at the very outer surfaces.Sections of moldings produced in this manner have been analyzed and havebeen found to have a dense, thin, outer layer, with the density of thefoam decreasing rapidly toward the core.

A problem which has plagued producers of such closed mold polyurethanemoldings, however, is the appearance of voids or surface imperfectionsin those portions of the mold into which the polyurethane mix wasexpanded, i.e., above the pour line of the liquid polyurethane mix. Theproblem has been alleviated to some degree by the provision of ventholes, i.e., holes of about one-sixteenth to one-eighth inch indiameter, in the mold. This solution is not entirely satisfactorybecause sufficient vent holes cannot be provided and because the ventholes cause surface imperfections in the finished molding. Voids andsurface imperfections are not generally encountered below the pour line.

Accordingly, it is the general object of this invention to overcome theproblems of the prior art and to provide a method for producing closedmold, foam polyurethane moldings having a uniform surface skin.

Another and more particular object of this invention is to provide amethod of fabricating foam polyurethane moldings in a closed mold whichmoldings are free of surface imperfections and voids.

Still further objects of this invention will be apparent from thefollowing disclosure.

Briefly, the method of this invention involves the flushing of theclosed mold with a gas capable of condensing on the mold walls ordissolving in the molding composition at the temperature and pressureconditions used in molding. It is desirable that the entire system,including the mold, the mixing chamber, and the pour lines, be flushedwith this gas. Any gas with the properties of condensability on the moldwalls or solubility in the molding composition at temperatures andpressures employed in molding operations is operative. A gas which istoo easily condensable on the surfaces of the mold is not preferredsince the condensation would take place before the molding operation.Ideally, a gas which remains in the gaseous form until the pressures ofexpansion of the molding in the closed mold cause it to condense on thesurface of the mold and/or to dissolve in the molding mix is preferredand sought for use in this invention. For instance, it has been foundthat mold temperatures of about 120 F are particularly suitable. What isdesired, therefore, is a gaseous material having an atmospheric boilingpoint below about F and easily condensable and/or soluble in the mixunder the conditions of pressure/temperature which predominate whenpolyurethane mix is expanded to fill the mold.

In general, original mold temperatures are chosen in relation to processvariables especially the composition of the urethane mix. Thetemperature is below about 250 F, preferably in the range of 1 10 to F,and below the temperature of the exotherming polyurethane mix. The vaporflushing agent is chosen so that its atmospheric boiling point is lessthan the original mold temperature to avoid undesirable condensationprior to the expansion of the polyurethane mix.

A number of gases possess the necessary physical characteristics but,although operable, are not considered suitable because they areexplosive or are excessively toxic. Halogenated hydrocarbons especiallyfluorinated hydrocarbons are the preferred materials.Monofluorotrichloromethane is the most preferred gas because itrecondenses at desirable temperatures and redissolves best, isnon-flammable and non-toxic. Dichlorodifluoromethane and its mixtureswith monofluorotrichloromethane are also operable but are not as highlydesirable as monofluorotrichloromethane. Methylene chloride is asuitable non-fluoro halogenated hydrocarbon.

Production of foam polyurethane itself is well-known and, accordingly,does not constitute a part of this invention. Urethane polymers are mostcommonly prepared by reacting a diisocyanate with a hydroxyl-terminatedpolyether or polyester. Foaming ingredients are added in order toproduce a gas which expands polyurethane into a foam. These materialsfall into three basic classes. First, water may be added to thepolyurethane mix in order to react with the isocyanate to produce freeCO gas. The second method involves the addition of a finely dividedsolid material which breaks down or decomposes at the temperature ofmolding to release a gas. A third, and most preferred, method involvesthe addition of a liquid which is volatilized at the temperatures ofmolding. The method of this invention may be applied to moldings whichuse any of the three methods to accomplish foaming.

The volatile material or blowing agent utilized in the last mentionedmethod may be of any of the materials known in the art. Generally, thevolatile materials are organic compounds, preferably halogenated organiccompounds and most preferably chlorinated-fluorinated organic materials.Monofluorotrichloromethane and dichlorodifluoromethane each are suitablebut a particularly preferred blowing agent comprises a mixture oftrichloromonofluoromethane and dichlorodifluoromethane in a weight ratioof from 6:1 to 12: I.

In addition to the foaming method, there are a variety of techniqueswhich can be employed in preparation of polyurethanes. The three basictechniques currently employed are classified as a complete prepolymer, aquasi-prepolymer, and a one-shot system. In the complete prepolymersystem, all of the base resin is reacted with polyisocyanate, the systemhaving a slight excess of isocyanate. To this mixture is added anaccelerator such as an amine, and the foaming agent. In thequasi-prepolymer system, a portion of the resin is reacted with theisocyanate component with a sufficient excess of isocyanate beingemployed to react with the remainder of the resin when it is later addedalong with the amine accelerator and a blowing agent. When theconstituents are mixed, they react to produce the urethane linkage, theblowing agent necessary to foam the reactant mass being triggered by theexothermic heat of reaction, or being produced chemically by theinteraction of isocyanate with water to produce CO gas.

In the one-shot system, there is no prereaction of resin and isocyanate.The method simply involves the mixture at one time of all of the rawmaterials under very carefully controlled conditions to form a finishedfoam. The present invention is easily applicable to any of theabove-described systems. The examples, however, refer to thequasi-prepolymer method.

Generally, the present invention is utilized in a closed system, i.e., asystem comprising a mixing chamber and a closed mold with a means ofconnecting the mixing chamber and the mold. The mold has a fixed volumeand is closable before it has been charged with the polyurethanereaction mix.

The mold can be vented in difficult to fill areas. The vents areone-sixteenth to one-eighth inch holes (e.g., three thirtyseconds)drilled through the mold and open to the atmosphere.

The sequence of process steps preferably employed in the presentinvention is as follows:

1. Flush the closed system (vents open) with a suitable gas in a volumeequal to about one to four times the free volume of the closed system,

2. close the system to the atmosphere, (vents open) supply thepolyurethane mix components at the proper temperature into the mixingchamber and mix,

. transfer the polyurethane mix charge to the mold, via the connectingmeans between the mix chamber and the mold,

5. close the connection between the mix chamber and the mold,

6. allow the polyurethane foam mixture to expand to fill the mold,

7. open the mold and remove the molding.

It is desirable that in the flushing operation, most of the air isremoved from the system; i.e., not more than about 5 percent air shouldremain. The above-indicated volumes of gas necessary to accomplish thispurpose reflect amounts which have been found to be satisfactory. It iswithin the skill of the art to determine the minimum amount of flush gasrequired.

The invention may be better understood by reference to the followingexamples, in which all parts and percentages are by weight unlessotherwise indicated. These examples are designed to teach those skilledin the art how to practice the invention and to represent the best modecontemplated for carrying out the present invention, and are notintended to limit the scope of the invention in any manner.

EXAMPLE I This is a control example and illustrates the preparation of apolyurethane molding without the use of the vapor flushing method ofthis invention.

The mold selected is one which is particularly difficult to completelyfill to produce surface-skinned, void-free polyurethane moldings;v Themold is aluminum having a large C- shaped cavity, 505 in. in volume, /4to 2 inches thick, and about 56 inches from one end to the other alongthe centerline of the C, with several small detailed contours near thetips of the arms of the C.

The urethane mix comprised a weight ratio of 49.5/50.5 of an A(polyisocyanate) component and B" (polyol) component.

The composition of the A component in parts by weight was 77.39 DiolA*/lsocyanate A** prepolymer having 25.75

percent free isocyanate 22.6] 80/20 mixture of 2,4-, and 2,6-toluenediisocyanate. Diol A is a dihydroxy polypropylene glycol of 400 MW.isocyanate A is a crude polymeric diphenyl methane diisocyanate,polymerized through the ortho position via methylene groups with two tofive isocyanate groups per molecule,

The composition of the B component in parts by weight was 83.7 of acommercially available propoxylated mixture of 75 percent a-methylglucoside and 25 percent glycerol having 435 hydroxyl number. 1.5 of acommercially available polymeric silicone polyol used as a surfactant.0.5 N,N,N ,N'-tetramethyl- 1 ,3-butanediamine.

14.3 monofluorotrichloromethane.

A commercially available foam mixing machine, provided with a port forflushing the head was used. The A (isocyanate) component was maintainedat F, the B component was maintained at 70 F. The mold was positionedwith the arms of the C pointed downward about 45 from the vertical. Thedesired amount of mix was poured into the unvented mold, which was atl20l23 F, at a pour rate of l8.5 lbs/min for 20.8 seconds. The chargedmold was allowed to stand for 3 minutes and the molding in the moldcured in an infra-red oven for 30 minutes.

The resulting molding weighed 2,818 g. Blisters up to inch diameter werenoted on the horizontal portion of the "C molding. Several smallerdeeper bubbles were found on the sides of the arms.

EXAMPLE 2 This example illustrates the use of an unvented mold and thevapor flushing of the mix chamber, pour hose and mold cavity inaccordance with the process of this invention.

The procedure of Example 1 was repeated (usingmonofluorotrichloromethane as a flushing agent). The mold was unvented(but not hermetically sealed) and the mix chamber, pour hose and moldcavity were flushed with g of monofluorotrichloromethane vapor in a 30second flush. At saturated vapor conditions, at atmospheric pressure,this is calculated to be 1,450 in. of vapor, or nearly 3 times the 505in. mold volume. The free volume of the remainder of the system is about10 percent of the mold volume.

A 2,680 g molding was obtained having excellent surface qualities excepta l/ l 6 inch corner void and a few small round bubbles.

Although the invention has been described in considerable detail withreference to certain preferred embodiments thereof, it will beunderstood that variations and modifications can be effected within thespirit and sco e of the invention as described above and as defined inthe appended claims.

What is claimed is:

1. A process for the production of foamed polyurethane moldingssubstantially free of surface voids or flaws comprising the steps of:

1. providing a fixed volume closed mold at a first mold temperature offrom about 1 10 to about 250 F;

2. flushing said mold with a vaporized liquid halogenated hydrocarbon involume equal to about one to four times the free volume of said mold;

. partially filling said mold containing said vaporized liquid with aliquid polyurethane molding composition containing a blowing agent, theremainder of said mold remaining filled with an atmosphere consisting ofat least 95 percent by volume of the vaporized liquid halogenatedhydrocarbon;

. expanding said molding composition to fill said mold at a second moldtemperature at least as great as said first temperature;

5. curing said molding composition into a solid foam, and;

6, recovering the resultant foamed molded product; said vaporized liquidhaving an atmospheric boiling point below said first mold temperatureand being capable of diffusion by condensation and/or solution into thepolyurethane at the temperature and pressure conditions existing insidethe mold at the mold surfaces during said expanding step.

2. The process of claim 1 in which said first mold temperature is in therange of about 1 10 to 160 F.

3. The process claim 2 wherein said temperature range is about 125 to F.

4. The process of claim 1 in which said vaporized liquid is selectedfrom the group consisting of trichloromonofluoromethane,dichlorodifluoromethane, mixtures thereof, and methylene chloride.

5. The process of claim 4 in which said vaporized liquid istiichloromonofluoromethane.

6. The process of claim 1 in which said blowing agent is a mixture oftn'chloromonofluoromethane and dichlorodifluoromethane in a weight ratioof 6:1 to 12:1.

7 A process for the production of foamed polyurethane moldings in aclosed mold comprising:

1. providing a closed system comprising a mixing chamber,

a fixed volume, closable mold at a first mold temperature of from about1 10 to about 250 F, means for interconnecting said mixing chamber andsaid mold;

2. flushing said closed system with a vaporized liquid halogenatedhydrocarbon in volume equal to about one to four times the free volumeof said closed system;

. supplying and mixing in said mixing chamber a liquid polyurethanemolding composition containing a blowing agent;

4. transferring via interconnecting means said polyurethane moldingcomposition to said mold to partially fill such mold, the remainder ofsaid mold remaining filled with an atmosphere consisting of at least 95percent by volume of the vaporized liquid halogenated hydrocarbon;

5. closing said interconnecting means between said mixing chamber andsaid mold;

6. expanding said molding composition to fill said mold at a second moldtemperature at least as great as said first temperature;

7. curing said molding composition into a solid foam, and;

8. recovering the resultant foamed molded product; said vaporized liquidhaving an atmospheric boiling point below said first mold temperatureand being capable of diffusion by condensation and/or solution into thepolyurethane at the temperature and pressure conditions existing insidethe mold surfaces during said expanding step 8. Process of claim 7wherein the first mold temperature is 1 10 to 160 F and said vaporizedliquid is selected from the group consisting oftrichloromonofluoromethane, dichlorodifluoromethane, mixtures thereofand methylene chloride.

9. The process of claim 7 in which said vaporized liquid is selectedfrom the group consisting of trichloromonofiuoromethane,dichlorodifluoromethane, mixtures thereof and methylene chloride.

10. The process of claim 7 wherein said blowing agent is a mixture oftrichloromonofluoromethane and dichlorodifluoromethane in a weight ratioof 6:1 to 12:1.

1 1. The process of claim 7 in which said first temperature of said moldis in the range of about to F.

2. flushing said mold with a vaporized liquid halogenated hydrocarbon involume equal to about one to four times the free volume of said mold; 2.The process of claim 1 in which said first mold temperature is in therange of about 110* to 160* F.
 2. flushing said closed system with avaporized liquid halogenated hydrocarbon in volume equal to about one tofour times the free volume of said closed system;
 3. supplying andmixing in said mixing chamber a liquid polyurethane molding compositioncontaining a blowing agent;
 3. The process claim 2 wherein saidtemperature range is about 125* to 145* F.
 3. partially filling saidmold containing said vaporized liquid with a liquid polyurethane moldingcomposition containing a blowing agent, the remainder of said moldremaining filled with an atmosphere consisting of at least 95 percent byvolume of the vaporized liquid halogenated hydrocarbon;
 4. expandingsaid molding composition to fill said mold at a second mold temperatureat least as great as said first temperature;
 4. The process of claim 1in which said vaporized liquid is selected from the group consisting oftrichloromonofluoromethane, dichlorodifluoromethane, mixtures thereof,and methylene chloride.
 4. transferring via interconnecting means saidpolyurethane molding composition to said mold to partially fill suchmold, the remainder of said mold remaining filled with an atmosphereconsisting of at least 95 percent by volume of the vaporized liquidhalogenated hydrocarbon;
 5. closing said interconnecting means betweensaid mixing chamber and said mold;
 5. The process of claim 4 in whichsaid vaporized liquid is trichloromonofluoromethane.
 5. curing saidmolding composition into a solid foam, and;
 6. recovering the resultantfoamed molded product; said vaporized liquid having an atmosphericboiling point below said first mold temperature and being capable ofdiffusion by condensation and/or solution into the polyurethane at thetemperature and pressure conditions existing inside the mold at the moldsurfaces during said expanding step.
 6. The process of claim 1 in whichsaid blowing agent is a mixture of trichloromonofluoromethane anddichlorodifluoromethane in a weight ratio of 6:1 to 12:1.
 6. expandingsaid molding composition to fill said mold at a second mold temperatureat least as great as said first temperature;
 7. curing said moldingcomposition into a solid foam, and;
 7. A process for the production offoamed polyurethane moldings in a closed mold comprising:
 8. recoveringthe resultant foamed molded product; said vaporized liquid having anatmospheric boiling point below said first mold temperature and beingcapable of diffusion by condensation and/or solution into thepolyurethane at the temperature and pressure conditions existing insidethe mold surfaces during said expanding step.
 8. Process of claim 7wherein the first mold temperature is 110* to 160* F and said vaporizedliquid is selected from the group consisting oftrichloromonofluoromethane, dichlorodifluoromethane, mixtures thereofand methylene chloride.
 9. The process of claim 7 in which saidvaporized liquid is selected from the group consisting oftrichloromonofluoromethane, dichlorodifluoromethane, mixtures thereofand methylene chloride.
 10. The process of claim 7 wherein said blowingagent is a mixture of trichloromonofluoromethane anddichlorodifluoromethane in a weight ratio of 6:1 to 12:1.
 11. Theprocess of claim 7 in which said first temperature of said mold is inthe range of about 125* to 145* F.